Ch 3.1 (Uniform)

CHAPTER 3 :

FORMS OF CORROSION

Forms of Corrosion:

PREFACE Corrosion occurs in several widely differing forms.

Classification is usually based on one of three factors:

Corrosion Classification

Nature of Corrodent

Appearance of the

Corroded Material

Mechanism of Corrosion

Forms of Corrosion:

PREFACE

Nature of the corrodent:

Corrosion can be classified as wet or dry.

A liquid or moisture is necessary for the wet corrosion

Dry corrosion usually involves reaction with high-temperature gases.

Forms of Corrosion:

PREFACE

Mechanism of corrosion:

Involves either electrochemical or direct chemical reactions.

Forms of Corrosion:

PREFACE

Appearance of the corroded metal:

Corrosion is either uniform and the metal corrodes at the same rate over the entire surface, or it is localized, in which case only small areas are affected.

T h e E i g h t F o r m s O f C o r r o s i o n

CORROSION FORMS

U N I F O R M

G A L V A N I C

C R E V I C E

P I T T I N G

I N T E R -

G R A N U L A R

S E L E C T I V E L E A C H N G

S T R E S S C O R R O S I O N

C R A C K I N G

E R O S I O N

Fontana, Mars G. & Greene, Norbert D., Corrosion Engineering, McGraw-Hill, New York, New York, 1967

Eight Forms of Corrosion

Source: http://corrosion-doctors.org/Localized/images/Introd1.gif

3.1 UNIFORM CORROSION

What is Uniform Corrosion?

Definition:

Uniform corrosion is a

corrosion attack with

almost constant

dissolution rates over the

whole surface (According to

DIN 50900 Norm).

Uniform corrosion of a mild steel coupon. Source: http://www.gewater.com/handbook/cooling_water_systems/fig24-2.jsp

What is Uniform Corrosion?

Other Definition: i. a type of corrosion attack (deterioration) that is more or

less uniformly distributed over the entire exposed surface of a metal

ii. uniform (or general) corrosion refers to the relatively uniform reduction of thickness over the surface of a corroding material.

iii. corrosion that proceeds at approximately the same rate over the exposed metal surface.

Cast irons and steels corrode uniformly when exposed to open atmospheres, soils and natural waters, leading to the rusty appearance.

2.1 Uniform Corrosion

occurs over the majority of the surface of a metal at a steady and often predictable rate.

Although it is unsightly its predictability facilitates easy control, the most basic method being to make the material thick enough to function for the lifetime of the component.

Cause 30% of failures

Some facts on Uniform Corrosion

1. Uniform corrosion is the most commonly found form of corrosion

2. Rust formed on low alloyed steel is a special form of uniform corrosion

3. Uniform corrosion occurs mainly on very active metals (low Reversible potential)

4. Noble metals (gold, platinum) are immune against uniform corrosion

5. Chromium, Titanium, Nickel, Stainless Steel are protected by a passive film

2.1 UNIFORM CORROSION

Another definition:

Uniform (or general) corrosion refers to the relatively uniform reduction of thickness over the surface of a corroding material.

Characteristics: It is relatively easy to measure, predict and

design against this type of corrosion damage.

While uniform corrosion may represent only a small fraction of industrial corrosion failures, the total tonnage wasted is generally regarded as the highest of all forms.

Uniform corrosion is usually controlled by selecting suitable materials, protective coatings, cathodic protection and corrosion inhibitors.

D.L. Graver (Ed.): Corrosion Data Survey-Metals Section, Sixth Edition, NACE International, Houston, 1985.

Relatively easy to monitor uniform corrosion; generally the simplest methods suffice (ER coupons, NDT techniques for thickness measurements).

Much data has been published that can be used for design purposes and estimating a "corrosion allowance".

In most practical cases, corrosive environments tend to differ from "textbook" cases (even small differences can be very significant).

Furthermore, actual uniform corrosion rates tend to vary with time; this variability is not accounted for by single "textbook values; therefore corrosion monitoring is advisable.

Caution: Unexpected rapid uniform corrosion failures can occur if the material's surface changes from the passive (low corrosion rate) to the active (high corrosion rate) state. The resultant increase in uniform corrosion rate is typically several orders of magnitude. This undesirable transition can occur if the passive surface film is disrupted by mechanical effects, flow rate changes, a chemical change in the environment etc. Real-time corrosion monitoring systems can detect such transitions.

A steel coupon corroded (rusted) uniformly over its entire surface after immersion in oxygen aerated water

In natural environment, O2 is the primarily cause of uniform corrosion of steels and other metals and alloys.

Same batch of coupons exposed to deaerated water retained their metallic appearance with no visible corrosion (rust)


TWO(2) types of uniform corrosion: 1. Uniform corrosion with laterally uniform active

dissolution rates The whole surface experiences a similar removal of material

Reaction kinetics are easy to obtain

2. Uniform corrosion with laterally heterogeneous reactions on the surface This type of attack results in the formation of craters with no

sharp edges

Heterogeneous uniform corrosion is the result of local difference in the corrosion condition due to materials heterogeneities or a difference in the solution aggressivity

2.1 Uniform Corrosion:

Most Common Damages

Uniform corrosion is found for example in water tubes, buildings (rebars), and bridges This is the most often occurring corrosion damage

but also the less dangerous because it is predictable

Damages are the result of: wrong material choice

wrong dimensioning of a structure

insufficient protective coating


Uniform corrosion on low alloyed steel


Mechanism of Uniform Corrosion

Requirement for uniform corrosion: Presence of a least a thin water layer (not visible in

the case of atmospheric corrosion) on the metallic surface

Homogeneous uniform corrosion Cathodic and anodic reaction occurs

simultaneously at the same location of the surface

an electron leaving the metallic surface is consumed by the cathodic reduction at the same place



Anodic metal dissolution

Me Me2+ + 2e-

Cathodic reduction

In acids pH < 4

2H+ + 2e- H2 (gas) (H Type)

In neutral and alkaline solution pH >4

O2 + 2H2O + 4e- 4OH- (O - Type)



Example: heterogeneous uniform corrosion

In bulk solution with convection, oxygen concentration is everywhere the same (homogeneous reaction)

In stagnant solutions, the oxygen concentration can vary laterally

Example of an aeration cell

How to prevent uniform corrosion?

Use thicker materials for corrosion allowance

Use paints or metallic coatings such as plating, galvanizing or anodizing

Use Corrosion inhibitors or modifying the environment

Use Cathodic protection (Sacrificial Anode or Impressed Current ICCP) and Anodic Protection


T H E E N D

Coming up Galvanic Corrosion

Documents

Ch 3.1 (Uniform)